The concept of preapplied adhesives has long been recognized as a desirable object and numerous attempts have been made to provide such adhesives. For example, attempts have been made to encapsulate two-part adhesives, such as epoxides, and to apply them to parts prior to assembly. While this has permitted pre-application of adhesives to parts, totally acceptable results have not been achieved. The encapsulation process is quite cumbersome and the attachment of capsules presents certain inherent problems, such as risk of loss of the capsules from the surface during storage or shipment. Also, mistreatment prior to use can lead to loss of adhesive due to premature rupturing of the capsules.
One attempt to solve the problems of preapplied encapsulated adhesives was to affix a coating or sheet material which does not migrate or flow off parts prior to the time of assembly, e.g., during normal storage and shipment. Adhesives of this sort are disclosed in U.S. Pat. Nos. 3,547,851 and 3,625,875. These patents disclose non-flowable anaerobic sheets or coatings which can be preapplied to a part and which will provide a convenient means of bonding to a mating surface. Compositions of these references are particularly useful on threaded fasteners, where the adhesive provides increased holding power and resistance to loosening of the mating surface, e.g., as in the case of nuts wound on bolts.
It is known in the prior art to use a polymerizable acrylate ester, a peroxy initiator, an accelerator and an aqueous system comprised of polyvinyl alcohol (see U.S. Pat. Nos. 3,814,156 and 3,489,599). This preapplied coating, however, does not always produce adequate adhesion to the substrate and sometimes peels easily or flakes off the part to which it has been preapplied. The results is lower bond strengths when the coated part is mated with another part. Additionally, these prior art compositions tend to be surface-sensitive and peeling is especially likely on inactive surfaces such as zinc, cadmium, and dichromates. As a direct result of such failure of the preapplied films of these adhesives compositions to adhere adequately to substrates, only some of the composition would be left to provide adhesion to a mated part, which then results in low torque values between a nut and bolt which have been coated with the prior art preapplied compositions. A further problem encountered in the prior art compositions was the interference of the binding system, in most instances polyvinyl alcohol, with the cure of the monomer.
Accordingly, there is a need for a stable adhesive composition which can be prepared and preapplied conveniently to a variety of metallic as well as non-metallic surfaces prior to the time of assembly, which will remain on the parts during normal storage and shipment, and which will cure upon mating with another part thereby imparting an effective and improved seal or bond. It is also desirable to have such a composition which is resistant to heat aging, which has an extended pot-life, and which has a binder system which allows for proper deposition and adherence of the preapplied adhesive composition.